In recent years, because of the increased awareness of environmental issues, solar cells have attracted attention as clean energy sources that cause no environmental pollution. Therefore, solar cells have been diligently studied for the use of solar energy as useful energy resources, and thereby have been commercialized.
There are various forms of solar cell elements. Typical examples of such solar cell elements include a crystalline silicon solar cell element, a polycrystalline silicon solar cell element, an amorphous silicon solar cell element, a copper indium selenide solar cell element, and a compound semiconductor solar cell element. Among them, the polycrystalline silicon solar cell element, the amorphous silicon solar cell element, and the compound semiconductor solar cell element are relatively inexpensive and can be manufactured in a large size. Therefore, they have been actively studied and developed in various fields. Further, among these solar cell elements, a thin-film solar cell element, which is typified by an amorphous silicon solar cell element that is obtained by laminating silicon on a conductive metal substrate and forming a transparent conducive layer on the laminate, is lightweight and excellent in the impact resistance and the flexibility. Therefore, the thin-film solar cell element has been regarded as a promising solar cell element as the form of solar cell in the future.
Among the solar cell modules, a simple one has such a structure that a sealing agent and a glass plate are successively laminated on both sides of a solar cell element. Since the glass plate is excellent in the transparency, the weatherproof property, and the friction resistance, it is commonly used as protection material on the solar-light-receiving side at the present time. However, for the non-light-receiving side that does not need to be transparent, various solar cell rear surface protection sheets (hereinafter also referred to as “rear surface protection sheets”) other than the glass plate have been proposed in consideration of the cost, the safety, and the workability (for example, Patent literature 1). Therefore, the glass plates are beginning to be replaced by rear surface protection sheets. Further, ethylene-vinyl acetate copolymer (hereinafter referred to as “EVA”) having high transparency and excellent moisture resistance is usually used for the sealing agent.
Examples of the rear surface protection sheet include (i) a single-layer film such as a polyester film, (ii) a film obtained by forming a vapor-deposition layer of a metal oxide or a nonmetal oxide on a polyester film or the like, and (iii) a multilayer film obtained by laminating films such as a polyester film, a fluorine-based film, an olefin film, and an aluminum foil.
Various properties can be added to a rear surface protection sheet having a multilayer structure because of its multilayer structure. For example, an insulating property can be added by using a polyester film. Further, a water-vapor barrier property can be added by using an aluminum foil (see Patent literatures 2 to 4). What kind of the rear surface protection sheet is actually used may be determined as appropriate depending on the product/use in which the solar cell module is used.
Among those various properties required for the rear surface protection sheet, an adhesive property with a sealing agent and an adhesive durability are fundamental and important required properties. If the adhesive property with the sealing agent is unsatisfactory, the rear surface protection sheet could be pealed and the solar cell cannot be protected from moisture and other external factors, and thus leading to the deterioration in the output performance of the solar cell.
As a method for ensuring the adhesive property with the sealing agent, there are known methods including (1) a method in which easy-adhesion treatment is carried out on a surface of the rear surface protection sheet that comes into contact with the sealing agent, and (2) a method in which a film having a high adhesive property with the sealing agent is used on a surface of the rear surface protection sheet that comes into contact with the sealing agent.
Examples of the above-described method (1) include surface treatment such as corona treatment, and easy-adhesion coating treatment in which an easy-adhesive is applied.
However, although the former method, i.e., the surface treatment such as corona treatment can ensure the initial adhesive property, there is a problem that the adhesive durability is poor.
Patent literatures 1, 5 and 6 disclose easy-adhesives that are used for the latter method, i.e., the easy-adhesion coating treatment.
Patent literature 5 discloses a coating liquid containing a cross-linking agent selected from a group consisting of a polymer containing an oxazoline group, a urea resin, a melamine resin, and an epoxy resin, and a resin component other than the cross-linking agent selected from a polyester resin or an acrylic resin whose glass transition point is 20-100° C. (see Claims 3 and 4 of the patent literature). More specifically, Patent literature 5 discloses an example in which a coating liquid containing an epoxy resin and an acrylic resin is used (see Example 5 of the patent literature).
However, the adhesive strength with the EVA sheet in this example is about 10-20 N for 20 mm width (i.e., 7.5-15 N for 15 mm width) (see Table 2 of the patent literature). Since the adhesive property between the sealing agent and the rear surface protection sheet has a significant influence on the deterioration in the output performance of the solar cell, the market demands a higher adhesive property and the reliability for the adhesive property under stricter conditions. The adhesive strength of about 20 N for 20 mm width cannot satisfy such demands in the market. Although the adhesive strength is improved in Patent literature 1, the market demands easy-adhesives having higher properties.
Patent literature 6 discloses such a configuration that an adhesion-improving layer in which at least one type of a resin selected from a group consisting of a polyester-based resin and a polyester polyurethane-based resin is cross-linked by a cross-linking agent composed of alkylated melamine or polyisocyanate on a polyester film is provided on a surface of a rear surface protection sheet that comes into contact with a sealing agent.
As for the above-described method (2) (method in which a film having a high adhesive property with the sealing agent is used on a surface of the rear surface protection sheet that comes into contact with the sealing agent). Patent literature 7, for example, discloses a method using polybutylene terephthalate (PET).
However, since the film like this usually has a thickness of several tens of micrometers, the cost becomes higher in comparison to the above-described easy-adhesion treatment.
Further, Patent literature 8, which is laid open after the previous application from which the present application claims priority was filed, discloses a back sheet for a solar cell module in which an adhesive layer composed of an acryl-based adhesive containing an acrylic polymer that is obtained by polymerizing a monomer component containing a monomer expressed by a general formula (I) shown below on a surface that is bonded to filler material (sealing agent) constituting the solar cell module is formed.CH2═C(R1)—CO—OZ  <Chemical 1>where R1 represents a hydrogen atom or a methyl group, and Z represents a hydrocarbon group having a carbon number of 4-25.
Further, Patent literature 9 discloses a rear surface protection sheet of a solar cell element including: a primer layer composed of a fluoric-based copolymer, an acrylic-based copolymer, or a polyurethane-based copolymer (polymer a); a polymeric monomer and/or an oligomer having at least one ethylene unsaturated group for photo-curing (monomer b); and/or a compound containing at least one ethylene unsaturated group and two or more isocyanate groups in the molecule (polyisocyanate c).